/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.sanselan.formats.png;

// should just use ints, not longs
public class PngCrc
{
	/* Table of CRCs of all 8-bit messages. */
	private final long crc_table[] = new long[256];

	/* Flag: has the table been computed? Initially false. */
	private boolean crc_table_computed = false;

	/* Make the table for a fast CRC. */
	private void make_crc_table()
	{
		long c;
		int n, k;

		for (n = 0; n < 256; n++)
		{
			c = n;
			for (k = 0; k < 8; k++)
			{
				if ((c & 1) != 0)

					c = 0xedb88320L ^ (c >> 1);
				else
					c = c >> 1;
			}
			crc_table[n] = c;
		}
		crc_table_computed = true;
	}

	/* Update a running CRC with the bytes buf[0..len-1]--the CRC
	 should be initialized to all 1's, and the transmitted value
	 is the 1's complement of the final running CRC (see the
	 crc() routine below)). */

	private final long update_crc(long crc, byte buf[])
	{
		long c = crc;
		int n;

		if (!crc_table_computed)
			make_crc_table();
		for (n = 0; n < buf.length; n++)
		{
			//			Debug.debug("crc[" + n + "]", c + " (" + Long.toHexString(c) + ")");

			c = crc_table[(int) ((c ^ buf[n]) & 0xff)] ^ (c >> 8);
		}
		return c;
	}

	/* Return the CRC of the bytes buf[0..len-1]. */
	public final int crc(byte buf[], int len)
	{
		return (int) (update_crc(0xffffffffL, buf) ^ 0xffffffffL);
	}

	public final long start_partial_crc(byte buf[], int len)
	{
		return update_crc(0xffffffffL, buf);
	}

	public final long continue_partial_crc(long old_crc, byte buf[], int len)
	{
		return update_crc(old_crc, buf);
	}

	public final long finish_partial_crc(long old_crc)
	{
		return (old_crc ^ 0xffffffffL);
	}
}